Tunable lasers have been widely used over the past several decades to measure the wavelength response of optical systems. Early applications were primarily in spectroscopy. Since the advent of wavelength division multiplexing in optical telecommunications, tunable laser applications have increased in the field of telecommunications components measurement.
FIGS. 1A and 1B show examples of wavelength-tunable tunable lasers. In each case, inputs can be applied to various actuators to effect a particular wavelength response in the optical output. FIG. 1A shows an example distributed feedback (DFB) laser (10). The laser cavity in this example is formed by the reflective facets of the laser diode 12. The output facet of the diode is slightly less reflective than the rear facet. Collimation optics 13 can be included to couple the output beam to an optical fiber. A periodic refractive index variation (grating) 11 is written in the laser diode during manufacture; the output wavelength is proportional to the period of this refractive index modulation. In this DFB example, the output wavelength can be tuned both by varying the temperature of the device via the thermo-electric cooler (TEC) 14, or by varying the laser diode current.
FIG. 1B shows an example external cavity laser 20 in a Littman-Metcalf configuration. The laser cavity is formed by the output facet of the laser diode 24 and the final reflection of the diffraction grating 22. This type of laser is wavelength tunable via variation of mirror 21 angle, optical cavity length, diode current, and cavity temperature. Rotation of the mirror and subsequent wavelength-selective reflection from the diffraction grating result in a very high degree of wavelength response compared to that due to variations in other inputs listed above. The geometry is organized such that cavity length is approximately preserved as the mirror is rotated, such that mode hops (instantaneous jumps in optical frequency) are mitigated. Collimation optics 23, 25 and an isolator 26 are used to focus and couple the laser beam to the fiber output, and ensure that stray reflections from the network do not enter the laser cavity, respectively.
The term “tunable laser” refers to wavelength tunable lasers. A tunable laser measurement system includes a laser and a measurement device and/or method for determining the wavelength of the laser. A tunable laser control system includes a mechanism for calculating an error signal corresponding to the difference between current wavelength and a desired setpoint. The error signal is used to drive the laser actuator(s).
A key component for a tunable laser measurement or controls system is the wavelength measurement itself. Various types of instruments and methods that can be used for this include the following, among others: Fizeau interferometer, grating spectrum analyzer, Fourier spectrometer, and etalon/wavelength reference combination. Additional methods include mapping wavelength response to actuator angle or position.
It is desirable that the measured wavelength signal exhibits the following characteristics:
1—directional: the wavelength measurement contains information about the direction of the laser tuning,
2—continuous: the measurement is available continuously, both temporally, and across the tuning range of the laser,
3—high accuracy: for applications such as spectroscopy and optical sensing, accuracy down to sub-pm levels is favorable and in some cases crucial,
4—high precision: accuracy is rarely meaningful unless precision is, at a maximum, equal to the accuracy of the control; many applications benefit from precision orders of magnitude better than accuracy,
5—absolute information: without absolute information, it is possible only to obtain a relative wavelength measurement; an unknown wavelength offset will exist between the wavelength measurement and actual wavelength,
6—no or negligible drift: many applications are sensitive to short- or long-term drift,
7—low latency: this requirement is particularly applicable to utilization of the wavelength measurement in a controls system; latency refers to the time taken between light exiting the laser and applying appropriate corrective signals to laser actuators. Time taken in calculation of the error signal contributes to the total latency, which latency is inversely proportional to feedback loop bandwidth; higher frequency control is available with decreasing latency.
A wavelength measurement based on any one of the measurement methods mentioned above does not fulfill these characteristics. Consider measurement of actuator angle and inference/calibration of angle to output wavelength for an external cavity laser, an example of which is shown in FIG. 1B. Given the strong correlation between the wavelength of the laser and the angle of the actuator tuning element, an approach to measuring the laser wavelength is to provide a high-resolution measurement of this angle using optical beam displacement or angle encoding. Wavelength is mapped to mirror actuator angle. In a controls application, an error signal is calculated based on a current wavelength setpoint and the wavelength inferred from the mirror actuator angle measurement. The angle of the mirror can be controlled by an electrical signal which can be applied to an actuator used to rotate or displace the tuning element. This electrical signal is determined from the error signal. Employing this method, the wavelength measurement is typically corrupted by a mismatch between actuator response and actual wavelength response and also by noise in angle detection. The result is a measurement that is neither high precision nor high accuracy. Indeed, to achieve tens-of-fm precision over an appreciable laser scan range (typically several tens or hundreds of nm), a signal to noise ratio (SNR) of 120-140 dB is required; this is well beyond the capability of such electro-mechanical systems. Moreover, drift in the wavelength measurement will likely at least be on the order of the precision of the measurement, and will therefore also be significant.
Consider a second possible error signal measurement technique that uses a combination of an etalon and wavelength reference. A wavelength reference (in the form of a gas cell, stabilized Bragg grating, etc.) provides absolute wavelength information. A gas cell provides high-accuracy, absolute wavelength information in the form of molecular absorption lines, which move negligibly with change in temperature or atmospheric pressure. An etalon provides a relative measurement of wavelength. Relative wavelength information is encoded by etalon fringe number or fraction thereof from a reference fringe. The etalon signal, due to its high resolution and linearity, provides accurate and precise wavelength information between gas cell absorption lines. Thus, the combination of the two provides high accuracy, high precision, absolute wavelength information.
Primary drawbacks of this approach, however, include a lack of continuity and directionality. FIG. 2A shows an example of a gas cell signal (three absorption lines shown with each line corresponding to a particular stable, known wavelength). FIG. 2B is an example of an etalon signal where each signal period corresponds to an interference fringe (i.e., free spectral range of the etalon in optical frequency). Both FIGS. 2A and 2B represent measured etalon/gas cell signals for a linear sweep in optical frequency (inverse of wavelength). Symmetry in the vicinity of the troughs or peaks of the etalon signal destroys the ability to distinguish the direction of wavelength change of the laser output; in addition, amplitude variation versus wavelength (sensitivity) varies periodically, with zero sensitivity at troughs or peaks of the etalon signal. Thus, the measurement is not continuous.
From a control systems standpoint, another drawback is that the etalon/reference wavelength measurement technique only has low latency for specific applications such as locking to a limited set of wavelengths (which set of wavelengths is limited to sloped areas of the etalon signal other than the peaks and troughs). The locking approach is employed for “step tuned” lasers that only need to achieve a discrete set of wavelengths. If a laser is controlled with an etalon signal, it is possible for it to lose lock to the current known fringe and lock to another fringe, the number of which is unknown. The fringe number may have increased or decreased one or several counts. Relative and absolute wavelength information is therefore lost in such a case. If the laser optical frequency noise (high frequency variations in output wavelength) is greater than the free spectral range (FSR) of the etalon (the period of the signal), this will lead to random, intermittent locking to unknown fringes, destroying not only absolute information but also the ability to lock to a relative wavelength.
Another drawback is that smooth tuning is not possible using the etalon signal since its sensitivity is periodic. The free spectral range of the etalon could be decreased by bringing the etalon interference signals closer together to achieve a smoother scan by effectively having a large number of very small discrete steps. However, the result of this periodic sensitivity for swept wavelength tuning is, at best, an oscillatory output wavelength pattern, with a period of oscillation the same as that of the etalon signal. Thus, for smooth laser tuning applications, post-processing is required to infer the actual output wavelength values as the wavelength setting for the laser changes. This post-processing introduces a very large latency and thus makes the etalon/gas cell combination impractical for locking to an arbitrary wavelength or for smooth tuning of the laser.
The relative optical frequency of a laser source can be precisely measured using a 3×3 coupler and a fiber interferometer, as described in detail in U.S. Pat. No. 6,426,496 B1 “High Precision Wavelength Monitor for Tunable Laser Systems,” Froggatt and Childers, the disclosure of which is incorporated herein by reference. This interferometer-based wavelength monitor provides an optical signal, the change in phase of which is proportional to the change in optical frequency of the laser source. It provides continuous, directional information, thus overcoming the aforementioned limitations of the etalon signal, while retaining its desirable qualities. Although the Froggatt et al patent describes a measurement system for a tunable laser, the measurement obtained lacks characteristics important to many wavelength measurement applications. The Froggatt et al patent does not teach using such a measurement system in a control system.
Indeed, the interferometer outlined in the Froggatt et al patent lacks several characteristics necessary for tunable laser control.
First, the interferometer lacks absolute information about the laser wavelength. It provides strictly a relative measurement, which alone cannot be used to infer the absolute wavelength, or to control a laser to a desired absolute wavelength. Second, due to the relative nature of the measurement, any errors in inferring wavelength from the wavemeter, however small, accumulate over time such that the resulting relative measurement is significantly corrupted. If the laser is tuned at a speed such that the sinusoidal signals from the interferometer exceed the Nyquist rate of the sampling system (or analog cutoff frequency in the case of an analog system) observing such signals, the fringe number (and therefore relative wavelength information) is lost, and error is accumulated. Vibration or mild shock of the laser can easily result in momentary tuning speeds in excess of the Nyquist rate or analog cutoff bandwidth, resulting in additional error. If a mode-hop occurs in the laser, (a common event for external cavity lasers), the wavelength monitor in Froggatt et al instantaneously accumulates a large error without any readily available way to correct that error. Further, if there is a loss of optical power, the laser monitor is unable to function properly. Upon restoration of power, there will be no information concerning the relationship of the current signal with that available before loss of power.
Third, the wavelength measurement drifts as the interferometer undergoes thermal changes. The interferometer is subject to thermal drift due to change in refractive index of the optical fiber with temperature, and also due to thermal expansion of the fiber. Indeed, in a fiber optic interferometer, thermal drift typically results in wavelength errors of approximately 10 pm/° C., regardless of the length difference in the two interferometer paths.
Thus, there is a need for a wavelength measurement system that overcomes these various deficiencies and that can generate an output which exhibits all of the desirable characteristics outlined above.